Integrated ultralong time constant time measurement device and fabrication process

ABSTRACT

An ultralong time constant time measurement device includes elementary capacitive elements that are connected in series. Each elementary capacitive element is formed by a stack of a first conductive region, a dielectric layer having a thickness suited for allowing charge to flow by direct tunneling effect, and a second conductive region. The first conductive region is housed in a trench extending from a front face of a semiconductor substrate down into the semiconductor substrate. The dielectric layer rests on the first face of the semiconductor substrate and in particular on a portion of the first conductive region in the trench. The second conductive region rests on the dielectric layer.

PRIORITY CLAIM

This application claims the priority benefit of French Application forPatent No. 1857842, filed on Aug. 31, 2018, the content of which ishereby incorporated by reference in its entirety to the maximum extentallowable by law.

TECHNICAL FIELD

Embodiments and implementations relate in general to integrated circuitsand more particularly to the production of an ultralong time constanttime measurement device.

BACKGROUND

Ultralong time constant (ULTC) time measurement circuits are qualifiedto make measurements with an application range on the order of tens ofminutes to several hours.

In a number of applications it is desirable to have information that isrepresentative of a time elapsed between two events, whether it be aprecise or approximate measurement thereof.

One example of an application relates to fraud prevention measures, inwhich a system is locked for long enough for it to be a deterrent. Thesepreventative measures apply to semi-invasive attacks, such as forexample analysis techniques employing fault injection, or tonon-invasive attacks, such as brute force attacks or side channelattacks. In general, this type of attack is based on self-learningmethods implementing numerous repeated iterations and precisesynchronizations.

In this context, locking the system for a duration of the order of anhour or of a few tens of minutes is enough to deter a fraudulent partyfrom employing such an approach.

This type of prevention by locking the system has the advantage of beingtemporary and non-destructive, for example in the case of a malfunctionor an operating error being detected as a fraud attempt.

Of course, it is desirable for the deactivation of the system not todisrupt the operation of measuring the duration of the lock.

An electronic device has already been proposed, for example in U.S. Pat.No. 8,872,177 (see also FR 2981190 A1), incorporated by reference, inwhich the time elapsed between two events is determined by measuring theresidual charge of a capacitive storage element that has been chargedbeforehand, which is connected to a series of charge flow capacitiveelements featuring leakages in their dielectric spaces. The residualcharge of the capacitive storage element is representative of the timeelapsed during discharging.

According to the teaching of U.S. Pat. No. 8,872,177, the dielectricspaces of the capacitive elements include a thick dielectric layer thatdoes not leak and leaky regions that are thinner, allowing charge toleak out by tunneling effect, and are thus produced with a view tocontrolling the dimensions of said leaky regions.

One drawback is that, in practice, the leaky regions are surrounded by athicker dielectric layer, in particular a silicon ONO(oxide-nitride-oxide) layer, which uses a non-negligible gross substratearea and imposes high-granularity design rules, i.e. the minimumdimensions thereof are relatively large and the embodiments thereof arebulky.

However, it is desirable to decrease the footprint of integratedcircuits.

There is accordingly a need for ultralong time constant time measurementdevices and measures that are capable of measuring a passage of time ofthe order of a few tens of minutes to several days, without requiring apower supply, substantially independently of temperature, andfurthermore having a minimized footprint.

SUMMARY

According to one aspect, what is proposed is an integrated ultralongtime constant time measurement device, comprising a plurality ofelementary capacitive elements in series, a capacitive storage elementthat is connected to one end of the series of elementary capacitiveelements and is capable of being charged, the series of elementarycapacitive elements being configured to discharge the charged capacitivestorage element and to deliver, to at least one node of the series, aphysical quantity that is representative of the discharging of thecapacitive storage element and of a duration that has elapsed betweenthe start of an operation of discharging the capacitive storage elementand the time at which the physical quantity is delivered, wherein eachelementary capacitive element comprises a stack of a first conductiveregion, of a dielectric layer, the thickness of which is suitable forallowing charge to flow by direct tunneling effect, and a secondconductive region, wherein the first conductive region is housed in atrench extending from a front face of a semiconductor substrate downinto the substrate while the dielectric layer rests on the front face ofthe substrate and the second conductive region rests on the dielectriclayer.

Thus, since the entire interface of the dielectric layer located betweenthe two conductive regions has a thickness that is suitable for allowingcharge to circulate by direct tunneling effect, no gross loss of area iscaused by such an embodiment. Specifically, an area that is 3.5 timessmaller with respect to a reference ultralong time constant timemeasurement device technology, mentioned above, has been obtained.

Furthermore, it is actually advantageous to measure a quantity that isrepresentative of the discharging of the capacitive storage element at apoint of at least one node of the series of elementary capacitiveelements rather than directly at the terminals of the capacitive storageelement.

According to one embodiment, said stack of the first conductive region,of the dielectric layer and of the second conductive region of eachelementary capacitive element is located facing a portion of said trenchhousing the first conductive region on said front face.

This allows the use of the area to be optimized.

According to one embodiment, the elementary capacitive elements areelectrically connected to one another in series, alternately either by asecond conductive region that is common to two consecutive elementarycapacitive elements or by a first conductive region that is common totwo consecutive elementary capacitive elements.

This allows the use of the area to be optimized.

According to one embodiment in which the semiconductor substratecomprises an electrical isolation region extending vertically into thesubstrate from the front face, said trench housing the first conductiveregion of each elementary capacitive element passes through theelectrical isolation region.

This makes it possible for example to prevent current leakages into thesubstrate without providing another thickness of dielectric facing thesubstrate for this purpose.

According to one embodiment, the plurality of elementary capacitiveelements in series is located in a semiconductor well that is housed inthe substrate and includes a first contact and a second contact, whichcontacts are electrically connected by an electrical path through thewell comprising a section that is located between the bottom of a trenchand the bottom of the well, and the device further includes a detectioncircuit that is configured to detect an electrical discontinuity in thewell between the first contact and the second contact.

Thus, in the event that an electrical discontinuity in the well betweenthe first contact and the second contact is detected, which would berepresentative of the substrate being etched from the back face, thedetection circuit will be able for example to order a preventative ordestructive countermeasure against this etch.

What is proposed is a method for measuring a duration, comprising anoperation of charging the capacitive storage element of a device such asdefined above, an operation of discharging the charged capacitivestorage element through the series of elementary capacitive elements andobtaining, on at least one node of the series of elementary capacitiveelements, said physical quantity that is representative of thedischarging of the capacitive storage element and of said duration thathas elapsed between the start of an operation of discharging thecapacitive storage element and the time at which the physical quantityis obtained.

According to another aspect, what is proposed is a process forfabricating an integrated ultralong time constant time measurementdevice, comprising an operation of forming trenches extending from afront face of a semiconductor substrate down into the substrate; anoperation of forming first conductive regions that are housed in saidtrenches; an operation of forming dielectric layers resting on the frontface, the thicknesses of which are suitable for allowing charge to flowby direct tunneling effect; an operation of forming second conductiveregions resting on said dielectric layers, the respective stacks of afirst conductive region, of a dielectric layer and of a secondconductive region forming a plurality of elementary capacitive elementsin series, the process further comprising an operation of forming acapacitive storage element that is connected to one end of the series ofelementary capacitive elements.

According to one implementation, the operations of forming thedielectric layers and the second conductive regions are located facingrespective portions of the width of said trenches housing the firstconductive regions on said front face.

According to one implementation, said operations of forming the trenchesand the respective first conductive regions and said operations offorming the dielectric layers and the respective second conductiveregions are positioned relatively so as to form the plurality ofelementary capacitive elements that are electrically connected to oneanother in series, alternately either by a second conductive region thatis common to two consecutive elementary capacitive elements or by afirst conductive region that is common to two consecutive elementarycapacitive elements.

According to one implementation, the process comprises an operation offorming an electrical isolation region extending vertically into thesubstrate from the front face, and said operation of forming thetrenches housing the first conductive regions is implemented through theelectrical isolation region.

According to one implementation, the process comprises a prior operationof forming a semiconductor well in the substrate and an operation offorming a first contact and a second contact, which contacts areelectrically connected by an electrical path through the well comprisinga section that is located between the bottom of a trench and the bottomof the well, and the process further comprises an operation of detectingan electrical discontinuity in the well between the first contact andthe second contact.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention will become apparent onexamining the detailed description of wholly non-limiting embodimentsand modes of implementation, and the appended drawings in which:

FIG. 1 illustrates an embodiment of an ultralong time constant timemeasurement device;

FIG. 2 is a circuit diagram of an application of the embodiment shown inFIG. 1;

FIGS. 3 and 4 show variants of an ultralong time constant timemeasurement device that includes a detection circuit configured todetect an electrical discontinuity; and

FIG. 5 shows fabrication steps for making a time measurement device suchas shown in FIGS. 1 to 4.

DETAILED DESCRIPTION

FIG. 1 shows an example of one embodiment of an ultralong time constanttime measurement device 10.

The device 10 belongs to an integrated circuit produced on asemiconductor substrate 1, for example a p-doped silicon substrate. Inthis depiction, the device 10 is formed in a portion of the substrate 1that is doped with the conductivity type opposite that of the rest ofthe substrate. This doped portion forms a semiconductor well 2, namely asingle well; that being so, it is possible that reference will be madehereinafter to the term “substrate” in order to refer to a portion thatis located in the well 2, in particular with respect to the electricalisolation regions STI and the trenches TR.

An orthonormal coordinate system orients FIG. 1 in a vertical directionZ, a horizontal direction X and a direction Y that is perpendicular tothe sectional plane (XZ) of the figure.

The device 10 comprises a plurality of elementary capacitive elementsC31, C32, C33, . . . , C3 n, which are electrically connected in series.

Each elementary capacitive element C31-C3 n comprises a stack of a firstconductive region P1, of a dielectric layer DI and of a secondconductive region P2. For example, the first conductive regions P1 andthe second conductive regions P2 may be formed of conductivepolycrystalline silicon and the dielectric layers DI made of siliconoxide. The thickness of the dielectric layer DI is suitable, inparticular according to the permittivity of the material used, forallowing charge to circulate by direct tunneling effect, in particularin the conditions described below with reference to FIG. 2.

In each elementary capacitive element C31-C3 n, the first conductiveregion P1 is housed in a trench TR. Each trench TR extends from a frontface FA of the semiconductor substrate down (in the Z direction) intothe substrate while the dielectric layer DI rests on the front face FAof the substrate and the second conductive region P2 rests on thedielectric layer DI.

In this depiction, the front face FA is located in a plane XY.

Each trench TR includes here an insulating envelope OX on its flanks andits bottom.

Unlike in conventional technologies, such an embodiment may be obtainedby following design rules for logic portions of integrated circuits,which limit for example the width of a strip of second conductiveregions P2 to 0.04 μm and the spacing between two such strips to 0.10μm, and the width of a trench TR to 0.11 μm and the spacing between twotrenches to 0.11 μm. According to these design rules, the area of anembodiment of an elementary capacitive element is smaller than 0.1 μm²,which may represent an area that is 3.5 times smaller than the area ofan elementary capacitive element based on conventional technologies.

In the example shown in FIG. 1, the stack of the first conductive regionP1, of the dielectric layer DI and of the second conductive region P2 ofeach elementary capacitive element C31-C3 n is located facing a portionof the width of the trench TR housing the first conductive region P1.The width W of the “exposed” surface of the first conductive region P1located on the front face FA is meant here. Specifically, the dielectriclayer and the second conductive region P2 are stacked on peripheralportions in the direction X of the width W, at the surface of the firstconductive regions P1.

This allows, in this example, said plurality of elementary capacitiveelements to be electrically connected to one another in series whileoptimizing the amount of area that is consumed. The elementarycapacitive elements are connected alternately either by a secondconductive region P2 that is common to two consecutive elementarycapacitive elements C3 i-1, C3 i (where i is an integer such that2≤i≤n−1) or by a first conductive region P1 that is common to twoconsecutive elementary capacitive elements C3 i, C3 i+1 (where i is aninteger such that 2≤i≤n−1).

Specifically, the elementary capacitive element C31 is connected inseries with the elementary capacitive element C32 directly via thesecond conductive region P2 which is common thereto, i.e. here theregion P2 that is provided with a contact node F1. Next, the elementarycapacitive element C32 is connected in series with the elementarycapacitive element C33 directly via the first conductive region P1 whichis common thereto, i.e. the region P1 that is provided with a contactnode F2. The consecutive elementary capacitive elements are connected toone another in this manner via a common second conductive region P2 orfirst conductive region P1. Each node of the series, between twoconsecutive elementary capacitive elements C3 j, C3 j+1 (where j is aninteger such that 1≤j≤n−1), comprises here a contact node Fj that isintended to deliver measurements of the charge of the elementarycapacitive elements C31-C3 n.

It is possible to envisage a configuration in which the stack of thefirst conductive region P1, of the dielectric layer DI and of the secondconductive region P2 of each elementary capacitive element C31-C3 n islocated facing the entire width W, in the direction X, of the trench TRat the surface of the first conductive regions P1. Thus, the analogousstack of the next elementary capacitive element is offset along thedirection Y on the same first conductive region P1. For example, thisconfiguration defines a course taking the shape of crenulations, or ofsteps, or taking any other shape, when viewed from above in the planeXY.

The elementary capacitive elements C31-C3 n connected to one another inthis way form a series C3 of capacitive elements, also referred to as a“flow element C3”. A first contact node 17 is formed at one end of theseries C3 and a second contact node 18 is formed at the other end of theseries C3.

What is meant by “producing a contact node” is for example the metalsilicidation of a silicon film allowing ohmic coupling to be appliedthereto.

Furthermore, in this embodiment, the semiconductor substrate 1 comprisesan electrical isolation region STI extending vertically (in the Zdirection) into the substrate from the front face FA. The electricalisolation region STI is for example a shallow trench isolation. Thus,the trench TR housing the first conductive region P1 of each elementarycapacitive element passes through the electrical isolation region STI.

The trenches TR extend vertically down into the substrate 1 further thanthe electrical isolation regions STI. Thus, the isolating envelope OXmay for example be located only on the portion of the flanks of thetrench TR that is located facing the substrate 1.

FIG. 2 is a circuit diagram of one application of the device 10described above with reference to FIG. 1, the references that are commonto FIGS. 1 and 2 denoting the same elements.

The device 10 further comprises in particular a capacitive storageelement C1 that is capable of being charged and being connected to thefirst end 17 of the series C3 of elementary capacitive elements. Thisseries C3 of elementary capacitive elements is configured to dischargethe charged capacitive storage element C1 and to deliver, to at leastone node F1-Fn−1 of the series C3, a measurement that is representativeof the discharging of the capacitive storage element C1.

The device 10 thus comprises a first capacitive element C1, a firstelectrode 11 of which is connected to a floating node F and a secondelectrode 12 of which is connected to a terminal 13 for applying apotential. The device 10 also includes a second capacitive element C2, afirst electrode 14 of which is connected to the node F and a secondelectrode 15 of which is connected to a terminal 16 for applying apotential.

The circuit 10 comprises the series C3 of elementary capacitive elementsC31, C32, C33, . . . , C3 n, the first node 17 of which, at one end ofthe series C3, is connected to the node F, and the second node 18 ofwhich, at the other end of the series C3, is connected to a terminal 19for applying a potential. The dielectric space of each elementarycapacitive element C31-C3 n is designed, by virtue of its permittivityand/or by virtue of its thickness, to exhibit non-negligible leakagesover time. The capacitive element C1 has a charge-holding capacity thatis higher than the equivalent capacity of the series C3 and thecapacitive element C2 has a charge-holding capacity that is higher thanthat of the series C3 but lower than that of the element C1.

The inventors have observed that when elementary capacitive elements areconnected in series, the resistance seen by the electric charge, whenleaking by tunneling effect, increases in proportion to the number ofelementary capacitive elements, resulting in an exponential decrease inthe speed at which the storage element C1 is discharged. One role of thecapacitive element C1 (storage element) is to store electric charge.

One role of the series C3 of elementary capacitive elements C31-C3 n(flow element) is to discharge the storage element C1 relatively slowlywith respect to a direct connection of its electrode 11 to ground. Onerole of the capacitive element C2 is to allow charge to be injected intothe capacitive element C1 while avoiding the stress that would result,for the flow element C3, from directly charging the storage element C1by applying a supply voltage between the node F and the terminal 13.

In a step of initializing a charge-holding phase, the terminals 13 and19 are at a reference potential, for example ground potential. A highsupply potential (positive with respect to ground) is applied to theterminal 16, resulting in the capacitive element C1 charging.

As a variant, in order to charge the element C1, the terminal 19 may begrounded, and the terminals 16 and 13 placed at potentials that arepositive and negative, respectively, in relation to ground.

When the supply voltage is no longer being applied between the terminals16 and 13, for example when the circuit is no longer being supplied withpower, the storage element C1 is discharged in a controlled manner(relatively slowly) through the flow element C3. It will be noted that acontrolled discharge phase may also be provided when the circuit isstill being supplied with power.

During the discharge phase, the terminals 13, 16 and 19 may be leftfloating, or placed at one and the same reference potential, for exampleground potential.

In a read step, after a discharge phase, the residual charge of thestorage element C1 is measured (for the measurement operation the devicemust be supplied with power). The residual charge of the element C1 isconsidered to be representative of the time elapsed between the end ofthe initialization step and the read step.

A reinitialization (reset) step may, if necessary, be provided in orderto completely discharge the storage element C1 through the capacitiveelement C2. For this, the terminals 19 and 16 may be grounded and theterminal 13 placed at a high supply potential. As a variant, theterminal 19 may be grounded, and the terminals 13 and 16 placed atpotentials that are positive and negative, respectively, in relation toground.

It is also possible to provide an accelerated controlled discharge phasefor discharging the storage element C1 (through the flow element C3),for example for the purpose of testing the time measurement circuit. Forthis, the terminal 19 may be grounded, and the terminals 13 and 16 maybe biased at one and the same potential that is positive with respect tothe ground, for example a potential that is between the ground potentialand the high supply potential.

In the example of FIG. 2, a plurality of comparators CPj, where j is aninteger such that 1≤j≤n−1, are connected to intermediate nodes Fj of theflow element C3. In a read phase, each comparator CPj receives, asinput, the potential of the node Fj that is common to the capacitiveelements C3 j and C3 j+1 and compares this potential with a threshold.In one embodiment example, one and the same comparison threshold is setfor all of the comparators, for example a threshold that is lower thanthe potential taken by the node Fn−1 when the storage element C1 ischarged. Each comparator CPj delivers binary information over an outputterminal Oj. As the storage element C1 is discharged, the comparatorswill switch one after the other, starting with the comparator CPn−1. Thebinary word formed by the set of binary information available over theterminals Oj (n−1 bits) provides information relating to the state ofcharge of the storage element C1, and hence to the time elapsed from thestart of the discharge phase of the element C1. This measurement circuithas the advantage of being of simple design with respect to a circuitfor measuring residual charge that is directly connected to theterminals of the element C1.

In order for the circuit 10 to operate properly, the node F and theintermediate nodes Fj are preferably floating nodes, i.e. nodes that areseparated, by a dielectric space, from any terminal applying apotential, rather than being directly connected to an unisolated regionof the semiconductor substrate of the chip (in which case leakages intothe substrate could dominate with respect to leakages through the flowelement C3).

As an embodiment example, the dielectric spaces of the variouscapacitive elements C1, C2, C3 may be formed of silicon oxide. In thisexample, the thickness of the dielectric layer of the capacitive storageelement C1 may be between 150 and 200 Å, the thickness of the dielectriclayer of the capacitive element C2 may be between 70 and 100 Å and thethickness of the dielectric layers of the elementary capacitivecharge-flow elements C31-C3 n may be of the order of 20 Å.

In any case, by virtue of the thicknesses of dielectric that are used,the capacitive elements C1 and C2 exhibit parasitic leakages (leakagesthrough their dielectric spaces) that are negligible with respect to theflow element C3.

FIGS. 3 and 4 show two variants of one embodiment example, in which theultralong time constant time measurement device 10 includes a detectioncircuit DET that is configured to detect an electrical discontinuity inthe well 2 between a first contact 301/401 and a second contact 302/402.

Specifically, since the time measurement device may be intended for theprevention of fraud attempts by reverse engineering, such as attempts toretrieve proprietary information, this embodiment example addsadditional protection to the device, for example in the event that afraudulent party had detected this time measurement device and hadgrounded the structure so that it would discharge instantaneously.

In this embodiment, the device 10 is located in a semiconductor well 2that is housed in the semiconductor substrate 1.

In the variant of FIG. 3, the semiconductor well 2 is a single well,i.e. a well having a conductivity type that is the opposite of theconductivity type of the semiconductor substrate 1. The well 2 and thesubstrate 1 are thus electrically separated by a p-n junction thatdefines in particular the bottom of the well 2.

In the variant of FIG. 4, the semiconductor well 2 is a triple well,i.e. a well having a conductivity type that is the same as theconductivity type of the semiconductor substrate 1, for example p-typeconductivity. The well 2 and the substrate 1 are electrically separatedvertically by a buried semiconductor layer NISO of the oppositeconductivity type, for example n-type conductivity, and laterally bysemiconductor shafts NW that are also of the opposite conductivity type.Thus, p-n and n-p (or n-p and p-n according to entirely inverseconductivity types) junctions allow the well 2 and the substrate 1 to beelectrically isolated for positive and negative biases. The p-n junctionbetween the well 2 and the buried semiconductor region 5 defines thebottom of the well 2.

In both variants, the semiconductor well 2 includes a first contact301/401 and a second contact 302/402 that are electrically connected byan electrical path through the well 2. The electrical path passesbetween the bottom of the trenches TR housing the first conductiveregions P1 and the bottom of the respective well 2. The electrical paththerefore comprises a section that is located between the bottom of saidtrenches TR and the bottom of the well 2.

If an etch 310/410 from the back face of the substrate 1 is carried outdown to the vicinity of the bottom of the trenches TR, the electricalpath will be broken. Measuring the continuity between the first contact301/401 and the second contact 302/402 thus allows such an etch 310/410to be detected easily.

To this end, the detection circuit DET is configured to measure thecontinuity between the first contact 301/401 and the second contact302/102. The detection circuit DET is thus able to generate adiscontinuity-revealing signal, for example when an attack from the backface is carried out.

The discontinuity-revealing signal could for example be used to triggercountermeasure means for guarding against such an attack.

FIG. 5 schematically shows fabrication steps for a time measurementdevice such as described above with reference to FIGS. 1 to 4.

In an initial step 51, wells 2 for example have been prepared in asemiconductor substrate 1 having a front face FA.

A step 52 comprises an operation of forming an electrical isolationregion STI extending vertically into the substrate 1 from the front faceFA. For example, this step is implemented according to a process forforming shallow trench isolations.

A step 53 comprises an operation of forming trenches TR extending from afront face FA of the semiconductor substrate down into the substrate 1.The operation of forming the trenches TR is implemented through theelectrical isolation region STI.

Step 53 may comprise a dry etch for etching the trenches TR in the well2 of the substrate 1. The design rules governing this etch 53 may forexample define a minimum width of 0.11 μm for a trench and a minimumspacing of 0.11 μm between two trenches.

Step 53 may further comprise an operation of forming an insulatingenvelope OX at least on the portions of the bottom and of the flanks ofthe trench that are surrounded by the well 2.

A step 54 comprises an operation of forming first conductive regions P1that are housed in said trenches TR. The operation of forming the firstconductive region P1 may comprise an operation of depositingpolycrystalline silicon, followed by a chemical-mechanical polishingoperation in order to remove excess material beyond the level of thefront face FA.

A step 55 comprises an operation of forming the dielectric layers DIresting on the front face FA, in particular on a portion that is locatedfacing respective portions of the surface of the first conductiveregions P1 on the front face FA.

The thicknesses of the dielectric layers DI are suitable for allowingcharge to flow by direct tunneling effect; for example, the operation offorming the dielectric layer DI may comprise an operation of partiallyor completely growing an oxide layer for an MOS transistor gate that isintended for use in logic operations.

A step 56 comprises an operation of forming second conductive regions P2resting on said dielectric layers DI. The operation of forming thesecond conductive layer P2 may also comprise an operation of depositingpolycrystalline silicon followed by a dry etch (for example the sameetch as that used to define the MOS transistors intended for use inlogic operations). The design rules governing this operation 53 may forexample define a minimum width of 0.04 μm for a strip of a secondconductive region P2 and a minimum spacing of 0.10 μm between twostrips.

The respective stacks of a first conductive region P1, of a dielectriclayer DI and of a second conductive region P2 are formed so as to obtaina plurality of elementary capacitive elements C31-C3 n that areelectrically connected in series.

To this end, steps 53, 54, 55 and 56 may be arranged with respect to oneanother so as to form a plurality of elementary capacitive elementsC31-C3 n that are electrically connected to one another in series,alternately either by a second conductive region P2 that is common totwo consecutive elementary capacitive elements or by a first conductiveregion P1 that is common to two consecutive elementary capacitiveelements.

A step 57 further comprising an operation of forming a capacitivestorage element C1 that is capable of being charged and being connectedto a first end 17 of the series C3 of elementary capacitive elementsthus allows an ultralong time constant time measurement device 10 of thetype of that described above with reference to FIG. 2 to be obtained.

In operation, discharging the previously charged capacitive storageelement C1 through the series C3 of elementary capacitive elements andproviding a measurement that is representative of the discharging of thecapacitive storage element C1 on at least one node F1-Fn−1 of the seriesC3 allows a timespan of the order of a few tens of minutes to severaldays to be measured.

Furthermore, the process may also comprise a prior operation, forexample in step 51, of forming a semiconductor well 2 in the substrate 1and an operation of forming, in the well 2, a first contact 301, 401 anda second contact 302, 402 that are electrically connected by anelectrical path comprising a section that is located between the bottomof a trench TR and the bottom of the well 2. Thus, a step 58 maycomprise an operation DET of detecting an electrical discontinuity inthe well 2 between the first contact 301/401 and the second contact302/402 may allow an attack by etching the integrated circuit from theback face to be revealed.

Advantageously, the process may be implemented at the same time as aprocess for fabricating a buried vertical-gate transistor, for exampleone belonging to a memory cell of a non-volatile memory. Of course, thisimplementation is advantageous in the case that the integrated circuitis intended to include a buried vertical-gate transistor, the processdescribed above with reference to steps 51 to 58 being capable of beingimplemented independently.

In a non-volatile memory NVM portion of one and the same substrate 1that was prepared in the initial step 51, isolation regions STI areproduced in a step 62 at the same time as step 52.

A step 63, carried out at the same time as step 53, comprises anoperation of forming a vertical gate GV trench, intended to house aconductive vertical gate region P1, extending from the first face (4) ofthe substrate 1 down into the substrate 1. Step 63 comprises anoperation of forming a dielectric envelope on the bottom and the flanksof the vertical gate GV trench.

A step 64, carried out at the same time as step 54, comprises anoperation of forming a conductive gate region PG that is housed in saidvertical gate GV trench.

A buried vertical-gate transistor TEGV is thus obtained in a step 65,upon completion in particular of upstream or downstream steps ofimplanting a drain region at the site of the front face 4 of thesubstrate 1, at the edge of the vertical gate GV, and of an operation offorming a buried source region in the well 2, at the bottom of thevertical gate GV trench.

Furthermore, the invention is not limited to these embodiments butencompasses all variants thereof; for example, it would be possible toenvisage using a charge flow device of the proposed type in any circuitsuitable for holding electric charge in order to measure time, otherthan that described with reference to FIG. 2.

The invention claimed is:
 1. An ultralong time constant time measurementdevice, comprising: a plurality of elementary capacitive elementsconnected in a series; a capacitive storage element that is connected toone end of the series connected plurality of elementary capacitiveelements and is configured to be charged; wherein the series connectedplurality of elementary capacitive elements is configured to dischargethe charged capacitive storage element and to deliver, to at least onenode of the series connected plurality of elementary capacitiveelements, a physical quantity that is representative of the dischargingof the capacitive storage element and a duration that has elapsedbetween a start of an operation of discharging the capacitive storageelement and a time at which the physical quantity is delivered; whereineach elementary capacitive element comprises: a stack of a firstconductive region, a dielectric layer having a thickness that allowscharge to flow by direct tunneling effect and a second conductiveregion, wherein the first conductive region is housed in a trenchextending from a front face of a semiconductor substrate into thesemiconductor substrate, and wherein the dielectric layer rests on thefront face of the semiconductor substrate and the second conductiveregion rests on the dielectric layer.
 2. The device according to claim1, wherein said stack is located facing a portion of a width of saidtrench housing the first conductive region on said front face.
 3. Thedevice according to claim 1, wherein pairs of elementary capacitiveelements in the series connection are connected, alternately, by thesecond conductive region that is common to two consecutive elementarycapacitive elements and by the first conductive region that is common totwo consecutive elementary capacitive elements.
 4. The device accordingto claim 1, wherein the semiconductor substrate comprises an electricalisolation region extending vertically into the semiconductor substratefrom the first face, wherein said trench housing the first conductiveregion of each elementary capacitive element passes through theelectrical isolation region.
 5. The device according to claim 1, furthercomprising: a semiconductor well that is housed in the semiconductorsubstrate where said series connected plurality of elementary capacitiveelements are located; a first contact and a second contact which areelectrically connected by an electrical path through the semiconductorwell comprising a section that is located between a bottom of the trenchand a bottom of the semiconductor well; and a detection circuitconfigured to detect an electrical discontinuity in the semiconductorwell between the first contact and the second contact.
 6. The deviceaccording to claim 1, further comprising a comparator having an inputcoupled to said at least one node of the series connected plurality ofelementary capacitive elements, said comparator configured to comparethe physical quantity at said at least one node to a threshold.
 7. Thedevice according to claim 6, wherein an output from the comparator is abinary value indicative of said duration.
 8. The device according toclaim 1, wherein a pair of elementary capacitive elements in the seriesconnection is connected by one second conductive region that is commonto the elementary capacitive elements of said pair.
 9. The deviceaccording to claim 1, wherein a pair of elementary capacitive elementsin the series connection is connected by one first conductive regionthat is common to the elementary capacitive elements of said pair. 10.The device according to claim 1, wherein said at least one node of theseries connected plurality of elementary capacitive elements comprises afirst node and a second node, and further comprising: a first comparatorhaving an input coupled to said first node, said first comparatorconfigured to compare the physical quantity at said first node to afirst threshold; and a second comparator having an input coupled to saidsecond node, said second comparator configured to compare the physicalquantity at said second node to a second threshold.
 11. The deviceaccording to claim 10, wherein outputs from the first and secondcomparator provide a binary word indicative of said duration.